Lead alloys as a material for electrode structures for use in lead-acid accumulators are known per se from the prior art. Electrodes usually comprise a solid electrode structure which serves to receive a paste-like active electrode material. The electrode structure is usually configured as an electrode grid. From the prior art, however, also other forms have been known. In relation to the lead alloys, regardless of the actual form of the electrode structures, they are usually called electrode grid alloys.
Several factors play a role in selecting the lead alloy. On the one hand, it is required that the lead alloy can be processed into electrode structures in an economically sensible manner. Furthermore, the lead alloy must have a comparably good mechanical stability in order to be able to bear both its own comparably high weight and the weight of the electrode paste throughout the service life of the accumulator. Moreover, when used as intended in a lead-acid accumulator, the electrode structure is always in contact with a highly corrosive electrolyte, on the one hand, and with the corrosive components of the active electrode paste, on the other hand. Therefore, in addition to the properties mentioned above, the lead alloy must be corrosion-resistant.
In this context, alloys containing lead, calcium, and cerium have been known for instance from U.S. Pat. No. 2,860,969 A. The publication deals with overcoming the disadvantages of lead-antimony alloys as a material for electrode grids. Originally, antimony was used in lead alloys in order to provide mechanical stability to the alloy. The concretely proposed alloy containing PbCaSnCe provides calcium as a substitute for antimony in order to provide the required mechanical stability and to prevent the deposition of antimony on the negative plate due to positive grid corrosion. As is well known, the contamination of the negative active paste by antimony leads to increased water loss by electrolysis and sulphation of the negative plates.
The alloying component cerium, on the other hand, serves to improve the corrosion properties by refinement of the grain sizes. Grids with coarse-grained structures have webs and frames consisting of a few grains. In this case, the corrosive attack on grain boundaries quickly penetrates deeply into webs and frames. This kind of intergranular corrosion usually results in an early disintegration of grids.
The disadvantage of electrode structures made of the alloys mentioned above, however, is that during operation as intended, they have an inclination towards grid growth, which may lead to results from an impairment of the positive connection of the grid paste to considerable capacitance losses of the accumulator. The grid growth phenomenon is the result of a lack of creep resistance of the grid alloys increasing over the course of time. This irreversible loss of mechanical strength is also called “overageing”. Under these conditions, the increasing thickness of the corrosion layer generates an axial force which leads to a considerable expansion of the grid webs and frames.